1. Field of the Invention
The present invention relates to a pneumatic tire in which steering stability in a heavy load state applied such as at the time of high speed cornering is improved without deteriorating riding comfort.
2. Prior Art
In a conventional pneumatic tire, as schematically shown in FIG. 5, a profile shape of a bead portion a is formed to match a profile shape of an inner surface of a rim R. More specifically, an outer wall surface Sw of the bead portion a supported by a flange surface Rf of the rim R comprises a vertical rising surface portion s1 rising from a bead heel surface Sh, and a concave arc surface s2 extending from an outer end of the rising surface portion s1 in a form of concave arc. A radial height ja of the rising surface portions 1 from a bead baseline BL is substantially equal to a height jf of a vertical surface portion Rf1 in the flange surface Rf, and a radius of curvature ra of the concave arc surface s2 is set to a value as slightly large as about 115 to 120% of a radius of curvature rf of a curved surface portion Rf2 in the flange surface Rf.
In the case of a tire having such a bead profile shape, in a normal running state, the concave arc surface s2 and the curved surface portion Rf2 of the flange do not come into contact with each other almost at all. When they come into contact with each other in a heavy load state in which lateral G is applied strongly such as at the time of high speed cornering, its contact pressure is low, and its contact position is extremely low as low as near root of the curved surface portion Rf2 of the flange. That is, the binding force is not varied by the rim almost at all between the normal running state and the heavy load state. Thus, in the heavy load state, the cornering power becomes insufficient, and the steering stability can not be exhibited sufficiently.
An example of means for enhancing the cornering power is to enhance the rigidity of the bead portion itself by increasing the amount of bead apex rubber b shown in FIG. 5 and by providing a cord reinforcing layer c. However, this means has a problem that the vertical spring constant is increased and the riding comfort is deteriorated.